Hi,
I have made a low-pass filter for my LM4780 amplifier, but I just have one question. Is it really necessary to use both channels from the audio input to the low pass filter? I would guess that there are the same bass tones on both channels, since you can't recognize where low frequency sounds comes from.
But if it's needed. Isn't it just to use 2 simple non-inverting op-amp setups and then connect both output together to get audio from both channels?

That depends of the crossover frequency and whether or not your recordings have a stereo bass at low frequencies.

If using two op amps ,why not to use them as stereo ? Do you have only one woofer?
I you want to sum the L,R channels, it is easy to do it with few resistors prior a single op amp.(the op amp may not be even necessary).

That depends on the impedance of your source. Common design practice is to ensure a low impedance source by sending it through a buffer before heading into any filter circuits. If you do this you can use a pair of resistors to mix the right and left channels together before feeding your sub filters, taking into account the new input impedance set by the resistors. You should take signal from both left and right before the sub filter. Signal could be present in varying amounts from both channels, with greater than 80dB channel separation (much less if phono), though the usual mixing job at the studio will have a pretty good balance between channels in the sub region.

Thanks for the replies. I have set the crossover frequency to 150Hz. I'll think I use the solution with the pair of resistors.
What would you recommend the value of the resistors to be? I have plenty of gain with my active 2nd order low pass filter (10 gain). I'm thinking of around 2-6kOhms.

Something in that neighborhood should be okay, especially if you try to drive a filter directly with it. Current only flows between them when there's a difference, of course, but if you load a preamp or buffer from channel to channel, all your stereo difference gets loaded together at the series value. You could use two low pass sections and sum after that if you wanted. It all depends on what you think is most important...

Thanks for the replies. I have set the crossover frequency to 150Hz. I'll think I use the solution with the pair of resistors..

You were right when you said the ear can not locate low bass sounds. But 150Hz is well above that threshold. You can locate a 150Hz sound. Try it. Play a 150Hz sin wave.

Middle "C" on a piano is about 256 Hz, So 150Hz is less than eight white keys below that. It is still right in the prime range where most music is played, Not a very good place to cross over to a sub it it's music you are listen to. OK if the material is dialog and car crashes in action films, but not music.

There is some advantage to applying the low pass BEFORE the two sides are summed. Summing any two channels really does degrade a stereo signal. Summing two mid range sounds may produce a beat frequency that would pass through a low pass filter. The effect is not large foer normal music but the cost of a second filter is only a few dollars

Hi,
Don't you mean 12db/24db filter? 12th order seems to be a bit extreme?
Anyway, I took and used a 2nd order lowpass filter. When I then connected it to the input of my amplifier, it just went crazy! It just gave me crazy noise and so on. I have tried with both TL072 and OPA2134, but neither of them gave me any good results. Then I used the LM358 opamp, and that's the only opamp that works?!
I have used TL072 before to the same amplifier with positive results, but now it wont work at all?
I'm using a LM317 to provide +/-8V, and it's bypassed with 100uF,1uF and 100nF capacitors before going to the opamp.
My amplifier is LM4780 in bridge mode just as the datasheets shows: http://www.national.com/ds/LM/LM4780.pdf
The amplifier itself is running at +/-35V (without playing).

Any solutions to the problem?

EDIT:
Running at half the voltage made it work, but when I turn the volume just slightly up, it start making some clicking noise.

What freqeuncy should I then use? I'm thinking of setting the crossover to 100Hz instead. What do you think?
And I'll think I use the method with using resistors to get both channel to my filter.

The answer depends mostly on what your other speakers can do. What are you using for a center channels and left and right? Will those speakers reproduce sound down to 50Hz or do they go down only to 100Hz.

In an idea world yur other speakers would reproduce sounds down to about 40Hz and your sub would only have to cover the range of (say) 20 to 50 or so. but speakers that cover down to 40Hz are large and expensive. So in an effort to save space and money people use cheaper and smaller center, left and right speakers andcross the sub over at a higher frequency.

So, what are your other speakers? Have you bought them yet? If you already own them then your choise of sub-woffer crossover point is already determined. But if you have not yet decided then you can make trade offs for space and cost and select whatever crossover point you like.

But ideally you'd cross them over at 60 or less Hz. as you go higher they become less suited to music and the sub beconme locatable by ear. Justthink about the notes on a piano. Do yo haear the C below middel C as "bass" and do you want that in your sub or in the main speakers? What about the C two octaves below?